The invention relates to a hydrostatic transmission, in particular for driving vehicles.
A hydrostatic transmission according to the preamble of claim 1 is already known from WO 94/09293. In this case a hydraulic pump is connected to an adjustable hydraulic motor via a direction-of-travel valve and a brake valve. The displacement volume of the hydraulic motor can be adjusted by an adjusting device, which is actuated by a regulating valve. A shuttle valve is disposed between the direction-of-travel valve and the brake valve and supplies the working pressure produced by the hydraulic pump to the regulating valve via a control line. The displacement volume of the hydraulic motor is therefore adjusted in accordance with the working pressure produced by the hydraulic pump.
In order to prevent the hydraulic motor from being operated at an excessively high speed, a speed sensor is disposed at the driven shaft of the hydraulic motor, which sensor supplies a microcontroller with an appropriate electrical control signal. The microcontroller converts this to an electrical initiator signal, which is supplied to an electromagnetic initiator disposed at the regulating valve. If the speed at the driven shaft of the rotor exceeds a predetermined threshold value, the microcontroller acts via the regulating valve on the adjusting device so as to reduce the speed of the hydraulic motor.
As a change-under-load transmission with two change-speed gears is subordinate to the hydraulic motor, the control characteristic described above is overridden in accordance with the gear which is engaged at the change-under-load transmission.
However this known hydrostatic transmission entails the disadvantage of relatively sluggish actuation of the hydraulic motor, as the control pressure acting on the regulating valve is derived from the working lines. A further disadvantage lies in the fact that the protection which is provided to limit the speed of the hydraulic motor is relatively complex, as it requires an electronic speed sensor, an electronic microcontroller and an electromagnetic initiator. Expenditure is increased as a result of these electronic components.
The object of the invention is therefore to provide a hydrostatic transmission which enables the displacement volume of the hydraulic motor to be rapidly adapted with low inertia to the desired travelling speed and at the same time guarantees effective protection against an excessive hydraulic motor speed.
The invention is achieved by the characterising features of claim 1 in conjunction with the features constituting the type in question.
The invention is based on the recognition that a control pressure produced by a pilot control device can be used to actuate both the driving control valve and the regulating valve. In this respect the control pressure produced by the pilot control device directly defines the delivery and the delivery direction of the hydraulic medium from the hydraulic pump to the hydraulic motor and, on the other hand, simultaneously determines the displacement volume of the hydraulic motor. This dual function of the control pressure produced by the pilot control device increases integration and simplifies the structure of the hydrostatic transmission. The regulating valve which regulates the adjusting pressure for the adjusting device is simultaneously directly actuated by the control pressure instead of indirectly via the working pressure in the working lines. By acting manually on the initiator of the pilot control device, the operator can therefore directly influence, the absorption volume of the hydraulic motor.
A further recognition according to the invention lies in the fact that the pressure difference between the control pressure and the working pressure prevailing in the working lines can be used to actuate the regulating valve. Protection against an excessive hydraulic motor speed can be provided by the direct action of the working pressure at the hydraulic motor on the regulating valve. If an excessive pressure in the delivering working line threatens to overload the hydraulic motor, this excessive working pressure causes, via the regulating valve and the adjusting device, the hydraulic motor to change over to a greater absorption volume and thus a reduced speed.
Claims 2 to 11 comprise advantageous developments of the invention.
According to claim 2, a separate control line is in each case connected to the pilot control device for each device, and the control valve is actuated by the pressure difference between the two control lines. According to claim 3, the respective higher control pressure can be tapped from the two control lines via a shuttle valve or two appropriately disposed non-return valves in order to actuate the regulating valve. According to claim 4, a shuttle valve, which is connected to the working lines, or two appropriately disposed non-return valves may be provided in corresponding fashion to actuate the regulating valve with the respective higher working pressure. It is alternatively possible, according to claim 5, to use a change-over valve instead.
According to claim 6, the regulating valve is advantageously a 3/2-way valve, which is connected on the inlet side to the working lines and the hydraulic medium tank and on the outlet side to an adjusting chamber of the adjusting, device. According to claim 7, the working pressure is admitted to a second adjusting chamber.
According to claim 8, a brake valve is advantageously provided which is actuated by the pressure difference in two first working line sections connected to the driving control valve. The brake valve enables a deceleration torque to be transmitted to the hydraulic motor by the back pressure resulting in the working lines in overrun, for example when driving downhill. According to claim 9, the brake valve is connected on the outlet side to each of the two working lines via non-return valves. The brake valve may therefore be of a relatively simple configuration and used for both directions of travel in the same way. According to claim 10, the brake valve is preferably a 3/2-way valve.
According to claim 11, an adjusting valve, for example an inching valve, may be disposed between a pressure source and the pilot control device so that a creep speed or emergency stop operation is possible.